The OFDM (Orthogonal Frequency Division Multiplexing) system is a system for dividing information to be transmitted into multiple carriers (hereinafter referred to as subcarriers) which are orthogonal to one another, and for modulating each subcarrier. Specifically, an OFDM signal is generated by implementing the inverse fast Fourier transform (IFFT) on a symbol mapped in accordance with the multi-phase PSK modulation or the multilevel QAM modulation, to which each subcarrier is applied, at the transmitting end. On the other hand, the OFDM signal is demodulated by implementing the fast Fourier transform (FFT) at the receiving end. The OFDM signal with finite duration generated by the IFFT is hereinafter referred to as “a transmission symbol”.
Generally, when radio communications are conducted under an environment where the transmitting and receiving ends cannot directly see each other, an antenna at the receiving end receives multiple radio waves having reflection paths different from one another (multipath waves), which leads to difference in arrival time delay between radio waves. Accordingly, as shown in FIG. 1, a method for adding a guard interval to each transmission symbol at the transmitting end is used in order to absorb the difference in arrival time delay caused by the multipaths.
In other words, a guard interval is added before or after a transmission symbol, and the transmission symbol and the guard interval thereof are set to be a transmission unit. Thus, a frame has a configuration including multiple transmission units. When the difference in arrival time between a radio wave which arrives fastest and a radio wave which arrives latest falls within the time length of a guard interval, use of the guard interval makes it possible to suppress the influence of many radio waves even if the many radio waves are received by the receiving end.
However, when a reflected wave (a delayed wave) which exceeds the guard interval length occurs, interference from preceding and subsequent symbols in terms of time (hereinafter referred to as “intersymbol interference”) occurs. As a result, great distortion is caused in a received symbol, so that the communication quality deteriorates. The difference in arrival time is a phenomenon caused by the propagation environment of radio waves. Accordingly, it is conceivable that controlling the time delay itself is impossible. In addition, the difference in arrival time changes depending on the place and time range of communication. In other words, the difference in arrival time may fall within the guard interval or may exceed the guard interval.
As a first method to solve the problem, considered is a method for making the guard interval length long enough to exceed the difference in arrival time, by taking into account the maximum difference in time delay.
Additionally, as a second method, considered is a method in which an advanced signal processing is employed at the receiving end. As a third method, considered is a method for appropriately controlling a guard interval length in accordance with the propagation environment.
On the other hand, proposed is a method for decreasing the number of guard intervals by conducting communications using a signal where two of the same multicarrier data continues (please refer to Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-open No. 2004-56552